The invention relates to a heating element for heating boiling plates, hotplates and the like with heating conductors arranged in spiral manner. Such heating elements can be formed from an insulating support and a reception shell and, e.g. when heating glass ceramic cooking trays, they are arranged below the glass ceramic plate. Such heating elements are, for example, described in German Pat. No. 2,729,929. The heating conductors are then arranged in the form of wire coils on a disk or card made from insulating material and are located in a reception shell, whilst being surrounded at the bottom and sides by insulating material. There, a heating conductor is provided, which supplies a timing, continuously adjustable power control device with power pulses of different relative ontimes. The mechanical construction of these heating elements has proved very satisfactory. However, it is necessary to use a power control device, which involves a certain manufacturing expenditure and the timing operation is often prejudiced by a heating system with such a low thermal capacity. The heating conductors are spirally arranged on the insulating support and can, e.g. in accordance with Offenlegungsschrift No. 3,129,239, be laid in the form of a double spiral, in which parallel, juxtaposed branches of the same heating conductor are laid as a spiral, the conductor being given a return bend at its centre. The distribution of the heating conductor over the surface is relatively uniform. However, there is the disadvantage that initially the full mains voltage is applied to the parallel branches, so that short-circuiting occurs in the case of a spiral deformation or a foreign body reaching this point.
It is also known to arrange two different heating conductors concentrically to one another in such a way that they can be individually switched, a central area being surrounded by a larger annular area, in order to be able to increase and decrease the effective size of the hotplate.
In the case of hotplates with heating conductors embedded in their bottom surface, it is conventional practice to use a circuit having switch means defining seven selectable conditions, where three separately switchable heating conductors are placed in three parallel spiral paths or pitches, i.e. in the form of a three-path spiral and which can be switched parallel, individually and in series by a cam-operated switch. Six different power stages are formed which, together with the "off"-position form the "seven steps of operation". This circuit is extremely reliable, can be easily operated by the housewife and the associated switch is very simple and is manufacturable with great operating reliability.
However, the construction with three parallel spiral paths requires very careful arrangement of the spirals, which normally perform three turns. In addition, the wire thickness and the helix density must be especially adapted for the individual power levels, because the overall length of the wire helix is given by the length of the spiral ducts. Thus, the wires are too thin for many applications, particularly when the wires are mounted other than by the complete embedding of the helix.